Long Term Evolution (LTE) was introduced in release 8 of 3rd Generation Partnership Project (3GPP) which is a specification for 3rd generation mobile communication systems. LTE is a technique for mobile data transmission that aims to increase data transmission rates and decrease delays, among other things. LTE uses orthogonal frequency division multiple access (OFDMA) as its multiple access method in the downlink. The uplink uses single-carrier frequency division multiple access (SD-FDMA). 3GPP release 10 introduced a next version of LTE, named LTE Advanced (LTE-A), fulfilling 4th generation system requirements.
Both LTE and LTE Advanced may utilize a technique called time division duplex (TDD) for separating the transmission directions from the user to the base station and back. In TDD mode, the downlink and the uplink are on the same frequency and the separation occurs in the time domain, so that each direction in a call is assigned to specific timeslots.
Herein, the term “downlink” (DL) is used to refer to the link from the base station to the mobile device or user equipment (UE), and the term “uplink” (UL) is used to refer to the link from the mobile device or user equipment to the base station.
FIG. 4 illustrates the frame structure for LTE TDD. The uplink and downlink for LTE TDD are divided into radio frames 400, each of which is 10 ms in length. The radio frame 400 consists of two half-frames 411, 412, both of which are 5 ms long. The first half-frame 411 is further split into five subframes 420-424, each 1 ms long. Similarly, the second half-frame 412 is further split into five subframes 425-429, each 1 ms long. Subframes 420, 422-425, and 427-429 are reserved for either downlink or uplink data, whereas subframes 421 and 426 are so called “special” subframes that include three special fields: downlink pilot time slot (DwPTS), guard period (GP) and uplink pilot time slot (UpPTS). However, as discussed below, in some configurations subframe 426 may also be reserved for downlink data, with the subframe 421 being the only special subframe. All non-special subframes consist of two time slots, both 0.5 ms long.
TDD allows asymmetry of the uplink and downlink data rates, i.e. as the amount of uplink or downlink data increases, more communication capacity can be allocated, and as the traffic load becomes lighter, capacity can be taken away.
This asymmetry is implemented via seven different semi-static uplink-downlink configurations, illustrated below in Table 1:
TABLE 1Uplink/downlinkSubframe numberconfiguration01234567890DSUUUDSUUU1DSUUDDSUUD2DSUDDDSUDD3DSUUUDDDDD4DSUUDDDDDD5DSUDDDDDDD6DSUUUDSUUD
In Table 1, “D” indicates that downlink data is transmitted in this subframe, “U” indicates that uplink data is transmitted in this subframe, and “S” indicates that the special fields DwPTS, GP and UpPTS are transmitted in this subframe. As can be seen, the seven different uplink/downlink configurations 0-6 contain different ratios of uplink and downlink data, and allow asymmetric uplink and downlink data rates.
Alternative to TDD, Frequency Division Duplex (FDD) mode of operation may be used.
LTE-A also includes a feature called carrier aggregation (CA). The basic principle of CA is to extend the maximum bandwidth available in the UL or DL directions (or both) by aggregating multiple carriers called component carriers (CC) which are then jointly used for transmission to/from a single mobile terminal. These component carriers may be of different bandwidths, and may be in the same or different bands to provide maximum flexibility in utilizing available radio spectrum.
Recent releases of LTE-A have also introduced cross-carrier scheduling for carrier aggregation. Cross-carrier scheduling involves transmitting scheduling information/control signaling to the terminal on a different component carrier than the corresponding data transmission. Cross carrier scheduling can be configured per UE basis, and it may performed by adding a CIF (Carrier Indicator Field) to DCI. As is known in the art, DCI refers to Downlink Control Information, and it is carried in Physical Downlink Control Channel (PDCCH) which in turn is carried in first up to fourth OFDM symbols of a given subframe.
There are several possible use cases for cross-carrier scheduling. For example, it helps to reduce DL control overhead in some CCs since the control signaling can be transmitted elsewhere. Also, cross-carrier scheduling allows handling a case in which interference on the control region is considered as too high to allow robust control channel performance.
In case of extension carriers, there may be the case in which an extension carrier is configured without any DL control region. The motivation of having such control-less carrier may include reduced control overhead on the extension carrier or to move the control signaling to anther CC which is able to provide better link performance for the control channels.
For the above scenarios, an issue illustrated in FIG. 5 has been identified. In the FIG. 5, the UEs are configured with two CCs, and configured with TDD UL/DL configuration #1 (500) and #4 (550), respectively. This is considered a typical case since for the co-existence with legacy TDD systems such as TD-SCDMA, TDD UL/DL configuration #1 or #2 is considered necessary for the relevant band. In this example, there are some UEs that are configured to have cross-carrier scheduling of secondary serving cell (Scell) (the cell associated with TDD UL/DL configuration 550) from the primary serving cell (Pcell) (the cell associated with TDD UL/DL configuration 500). In general, when UEs are configured with such cross-carrier scheduling, they will only monitor UE-specific search space from the Pcell. However, since the subframes 508-509 marked with shadowing in FIG. 5 on the Pcell are UL subframes, it is not possible to transmit any DL or UL grants from these subframes on the Pcell, which means in this case the two corresponding DL subframes 558-559 (marked with shadowing) in Scell cannot be scheduled. It could be argued that for the DL subframes marked with shadowing, all UEs may not have to be scheduled there, but that eNB can schedule some other UEs that are configured with only one DL CC without cross-scheduling. This may be one way of reducing the TP loss of, not being able to schedule the UEs configured with two CCs. However, this is clearly not a satisfying solution, since for the CA-capable UEs such restriction would lead to less scheduling possibilities (thus reduced scheduling gain), and it also would result in less available Physical Downlink Shared Channel (PDSCH) resources. These restrictions would then result in overall system performance loss.
Furthermore, it is known that an extension carrier can be configured to be with or without PDCCH region as needed. Following such concept, similar with the issue above, if the Scell is an extension carrier (which is not a backwards-compatible CC) and is configured without any PDCCH region, then for the subframes marked with shadowing in FIG. 5, it is not possible for the UEs to receive any DL or UL grant corresponding to the PDSCH or Physical Uplink Shared Channel (PUSCH) on the Scell. Furthermore in this case during PDSCH de-mapping and decoding, UE will assume that there is no PDCCH region on the extension carrier (i.e. PDSCH will start from symbol #0 in the subframe). In this case, if a PDCCH were to be transmitted on the extension carrier due to lack of any DL subframe on the backwards-compatible CC, it would result in an error case in PDSCH decoding on the extension carrier.
Another relevant issue is the ambiguity on Physical Control Format Indicator Channel (PCFICH) value (which is used to indicate the number of symbols occupied by PDCCH in the subframe), if PDCCH is transmitted from Scell or the extension carrier instead. In LTE Release 10, when cross-carrier scheduling is configured, the value of the PCFICH on the cross-carrier scheduled cell would be semi-statically conveyed via a higher layer. This works for LTE Release 10 since in that case the amount of DL control needed in the cross-carrier scheduled CC will not change dynamically. One more motivation of having a semi-static PCFICH in LTE Release 10 for the cross-carrier scheduled cell is to avoid potential error cases due to high interference on the cell. On the other hand, on the Pcell the PCIFCH value is still dynamic since the number of UEs to be scheduled in a given DL subframe may not be known in advance. However for the scenario under discussed, the above considerations do not necessarily hold. For example, for the subframes marked with shadowing in FIG. 5, eNB may not be able to know accurately: a) the interference level in the control region in the Scell, and b) the number of UEs that are to be scheduled in a given DL subframe. Therefore, it is insufficient and inefficient to always use dynamic PCFICH on Scell or to have a higher layer configured PCFICH value.
Furthermore, there are severe cross-carrier scheduling issues in an unlicensed band CA scenario, in which the Pcell of licensed carriers can be potentially aggregated with Scell on unlicensed band carriers. If the Pcell in a licensed band runs out of control resources, the PDCCH for scheduling data for unlicensed Scell(s) has to be transmitted from these unlicensed Scells. However, the situation might be that not all the carriers in the unlicensed band are reliable enough to carry the PDCCH channel. For example, at time T1, CC1 in an unlicensed band is relatively more stable than other unlicensed band CCs, therefore only CC1 is configured with PDCCH control region, and CC2 and CC3 are extension carriers which are cross-scheduled from CC1. However, at time T2, the CC1 becomes less reliable, and it is more desirable to have CC3 to carry the control for this group of unlicensed band CCs. This type of reconfiguration might happen quite often due to unpredictable interference changes in unlicensed bands.
If one were to carry out this operation with current LTE Release 10 procedure, the radio resource control (RRC) signaling and medium access (MAC) signaling overhead and delay would be high, since a large amount of reconfiguration signaling would be necessary. For example:
1. First eNB needs to enable the PDCCH control region in CC3;
2. For CC1, change the cross-carrier scheduling from CC1 to CC3;
3. For CC2, change the cross-carrier scheduling from CC1 to CC3;
4. For CC3, change the cross-carrier scheduling from CC1 to CC3;
5. Then the control region in CC1 needs to be disabled.
As can be seen, it is critical to reduce the signaling overhead and delay for the cross-carrier scheduling reconfiguration for unlicensed band CA.
Therefore, an object of the present invention is to alleviate the problems described above and to introduce a solution that allows flexible disabling/enabling of cross-carrier scheduling in carrier-aggregated wireless data transmission.